Microwave drying and heating of food, wood, ceramics, textile and pharmaceutical products is a common industrial practice. The microwave energy directly interacts with the processed materials to raise the interior temperature and significantly reduces treatment times as compared to conventional hot-water immersion or heated air methods.
Dielectric heating, which includes both radio frequency and microwave heating, has been investigated for insect disinfestation in stored grains and their derived products (Webber, Wangner, Pearson, 1946; Halverson, Burkholder, Bigelow, Norsheim, Misenheimer, 1996; Shayesteh & Barthakur, 1996; Nelson, 2001). Microwave treatment kills insects inside and outside grain kernels and also affects reproduction of survivors (Hamid, Kashyap, Cauwenberghe, 1968; Vadivambal, Jayas, White, 2007). However, the non-uniform temperature distribution of the processed materials limits its application on an industrial scale (Vadivambal & Jayas, 2007).
Researchers have investigated temperature distribution of food materials heated by microwaves (Sakai, Wang, Toba, Watanabe, 2004; Manickavasagan, Jayas, White, 2006; Gunasekaran & Yang, 2007; Nelson & Trabelsi, 2012). The uneven temperature distribution depends on many factors such as shape, volume, mass, chemical composition and moisture content of the processed food materials (Kelen, Ress, Nagy, Pallai, Pintye-Hodi, 2006). Dielectric properties of biological materials also depend on microwave frequency and composition, moisture content and temperature of the treated materials. The density of the processed material also affects its dielectric properties because different amounts of mass have different interactions with the electromagnetic fields. This is especially notable with particulate dielectric materials such as cereal grains, oilseeds, pulses (collectively referred to as grains) and soils. Consequently, temperature distribution is different from product to product, from equipment to equipment (Kelen et al., 2006) and from treatment to treatment.
The non-uniformity of microwave heating results in some hot spots with considerably higher temperature than average temperature. Manickavasagan et al. (2006) found that the temperature on the surface of treated wheat bulk was non-uniform. Furthermore, the maximum and minimum temperature and moisture distribution inside the treated wheat of different moisture contents by microwaves has not been reported.
There are several possible solutions to avoid the undesired non-uniformity of temperature and moisture content (Kelen et al., 2006), for example, intensification of the mixer motion and reduction of the microwave power. An increase in mixer motion can only decrease the non-uniformity to a certain degree, and domestic microwave ovens are usually equipped with a rotating plate. The use of microwaves for disinfestation of insect pests is based on the dielectric heating effect produced in grain and insect bodies. Insect bodies have higher moisture contents than stored grains and products derived from grains. The higher the water content, the higher the values of dielectric properties of a material. The dielectric loss factor of the insects was found to be much higher than that of stored grain and products (Nelson, 1991; Nelson & Whitney, 1960; Nelson, Stetson, Rhine, 1966; Ikediala, Tang, Wig, 2000; Rashkovan et al., 2003; Alfaifi, Tang, Rasco, Sablani, Jiao, 2013). The higher values of dielectric properties might result in higher temperatures inside insect bodies. The temperature difference between no-moveable model insect and walnut was about 4.6 to 6° C. (Wang, Tang, Johnson, Cavalieri, 2013). However, an insect body is smaller than treated grain kernels and heat inside an insect body might be transferred quickly to grain kernels during microwave treatment (Wang et al., 2013). Mobile insects such as adult rusty grain beetles, Cryptolestes ferrugineus (Stephens) (Coleoptera: Laemophloeidae), a common insect pest of moist stored grain throughout the world, are very sensitive to temperature and will escape when temperatures are high (Jian, Jayas, White, 2002).
During microwave treatment, insects move towards the surface from inside the treated wheat (Shayesteh & Barthakur, 1996). Insects were more active within a microwave field (Shayesteh & Barthakur, 1996). Heat plus light are used in Berlese funnel extraction of insects from stored grain (Berlese 1905). Therefore, microwave treatment might be used to force insects out without killing them. There is no study using microwave to expel insects.
Annually, over 2.6 billion tonnes of grains, including cereals, oilseeds and pulses, are grown. These products are then stored until they can be used by consumers. Most countries do not report how much grain is lost annually in storage and is not used for human consumption, but based on observations, these numbers are likely to be high (Jayas 2013). Losses occur when grain decays or is infested by pests, fungi or microbes, and physical losses are only part of the equation. Losses can also be economic, resulting from low prices and lack of access to markets for poor quality grain, or malnutrition and death of human being and animals, arising from poor quality or contaminated food. In some countries, grain cannot be sold if it is infested with insects (Canada, Australia) or grain is downgraded (USA)
Information on insect infestation inside stored grain is required for safe grain storage, before and after insect control, before grain loading and unloading, and during domestic and international sales and transportation. Detecting insect infestation is difficult under most grain storage conditions.
Storage management greatly influences pest development and control within grain. It encompasses decisions upon the location of stores, storage periods and the quality control objectives for stored commodities. All of these have substantial implications for pest management and are components of the complex interactive network of factors affecting loss reductions in grain storage.
Grain monitoring plays an imperative role in the development of pests in grain. For example, if we can monitor grain effectively, we can develop a sound integrated pest management (IPM) program and better solutions to grain storage problems. Therefore, information on insect infestation inside grain is required for safe grain storage, before and after chemical and physical insect control, before grain loading and unloading, and during domestic and international trading negotiation and transportation. To detect insect infestation inside grain held in grain storage structures such as bins (silos), trucks, railcars, ships, vessels, containers, and bags, grains are usually sampled using mechanical and manual devices. The sampled grain is further investigated inside the laboratory using the naked eye and/or equipment. However, the shaking and naked eye method cannot detect hidden insects. To detect both hidden and external insects, the following methods are suggested: flotation and cracking (Brader et al., 2002), acoustic sensor (Gutierez et al., 2010), immunoassay (Krizkova-Kudlikova and Hubert, 2008; Atui et al., 2007), single kernel characterization (SKCS) (Pearson et al., 2003), electrical conductance (Brabec et al., 2010; Pearson and Brabec, 2007), near-infrared hyperspectral imaging (Singh et al., 2009), and soft −X ray roentgenography (Karunakaran, et al. 2004, Nawrocka et al., 2012). None of these recommended methods is commercialized or used by grain industry due to some of the following disadvantages: high cost, limited capacity, intensive labor, time consuming, and low accuracy.
The most used and commercialized process is the Berlese funnel method and this method is also widely used in separation of insects from soil and other biomass materials. The Berlese funnel method uses an incandescent light bulb to provide heat that forces insects to exit grain kernels (Berlese, 1905). Problems with this method include that it takes at least 6 hours; collects only 30-70% of insects in the grain samples (Minkevich et al., 2002); incandescent light bulbs are being phased-out; and it requires extensive space for running multiple samples. Thin or multi layers soil, grains, processed granular foods, and breakfast cereals were suggested to increase the accuracy and efficiency of the Berlese funnel method. However, these modified methods did not increase its detection accuracy (Minkevich et al., 2002). The modified methods also require at least 1 h. Therefore, the Berlese funnel method cannot be used when immediately (e.g. in less than half hour) evaluation of grain infestation is required. Before grain loading and unloading at farms, elevators, and terminals, further actions will depend on whether there are insects inside the grain samples. In Canada, the Beriese funnel method is used for detection of infestation and infested grain must be treated (Canada Grain Act, 1994). Waiting for the determination of insect infestation increases handling and transportation cost. Therefore, a rapid method to detect insects inside sampled grain with larger capacity and higher accuracy is required.